The present invention relates to ultraviolet (UV) radiation curable polysilarylene-polysiloxane copolymers and compositions made therefrom. More particularly, the present invention relates to UV-curable block copolymers (terpolymers) having silarylenesiloxane, diorganosiloxane and acryloxy-functional siloxane units. The UV-curable copolymers of the present invention can be crosslinked in the presence of a suitable photoinitiator to provide highly transparent, self-bonding, dirt repellent, tough and solvent resistant compositions useful as conformal coatings, electrical insulation, gasketing and the like.
Acryloxy-functional polysiloxanes are well known in the art. For example, Merker in U.S. Pat. No. 2,793,223 discloses acryloxy methyl siloxanes having the formula ##STR4## where R is hydrogen or methyl, R' is a monovalent hydrocarbon radical, and n has a value from 1 to 2, inclusive. Such acryloxy substituted siloxanes can be copolymerized with siloxanes of the unit formula ##STR5## where R" is a monovalent hydrocarbon radical, an acetoxymethyl radical or a halogenated monovalent hydrocarbon radical, and m has an average value from 1 to 3, inclusive.
Merker, in U.S. Pat. No. 2,922,806, expands upon U.S. Pat. No. 2,793,223 in that the acryloxy or methacryloxy groups are attached to silicon through a divalent alkylene radical having from 2 to 4 carbon atoms.
In U.S. Pat. No. 2,956,044 Merker further teaches that copolymers of acrylate-functional organosilicon units and organic vinylic units (e.g. styrene) exhibit many of the properties of organic vinylic compounds as well as conventional organopolysiloxane compounds. An example of a resulting copolymer would be a copolymer having the general formula ##STR6## where R, R' and R" are as previously defined.
Liu, in U.S. patent application Ser. No. 688,489, filed Jan. 3, 1985, and assigned to the same assignee as the present invention, discloses UV-curable block copolymers comprising (a) at least one block consisting essentially of from about 6 to about 120 units of the formula ##STR7##
(b) at least one block consisting essentially of from about 3 to about 160 units of the formula ##STR8## and
(c) at least one block consisting essentially of from about 0.1 to about 20 mole percent of (a) and (b) of units of the formula ##STR9## where R is an independently selected alkyl radical having 1 to 5 carbon atoms, aryl radical having 6 to 10 carbon atoms or aralkyl radical having 6 to 10 carbon atoms, R.sup.1 is an aryl or aralkyl radical having 6 to 10 carbon atoms, and R.sup.3 is an acrylic functional radical of the formula ##STR10## where each R.sup.4 is the same or different and represents hydrogen atoms or monovalent substituted or unsubstituted hydrocarbon radicals having from 1 to 12 carbon atoms, and R.sup.5 is a divalent substituuted or unsubstituted hydrocarbon radical having from 2 to 12 carbon atoms.
While compositions prepared in accordance with the teachings of the foregoing disclosures generally exhibit good physical properties, it nevertheless is desirable to provide even better compositions which are curable upon exposure to ultraviolet radiation to transparent, self-bonding, dirt repellent and solvent resistant materials. The present applicants have found that such improved UV-curable copolymers can be prepared by incorporating silarylene units in the polymer chain. Even more improved copolymers result when the copolymers of the present invention are block copolymers rather than random copolymers.
Silarylenesiloxane block copolymers are described in the prior art, for example, as in Merker, U.S. Pat. No. 3,202,634, which effect curing by exposing the copolymer to ionizing radiation or by heating the copolymer in the presence of well known catalysts.
Omietanski, U.S. Pat. No. 3,287,310, describes heat curable random copolymers comprised of (a) from 5 to 95 mole percent units of the formula ##STR11##
(b) from 5 to 95 mole percent units of the formula ##STR12##
(c) from 0 to 25 mole percent units of the formula ##STR13## and
(d) from 0 to 1 mole percent units of the formula ##STR14## where R is a monovalent hydrocarbon radical and R' is an olefinically unsaturated monovalent hydrocarbon radical such as vinyl, allyl, butenyl, cyclohexenyl and the like.
Nowhere, however, does the prior art disclose or suggest ultraviolet radiation curable polyarylene-polysiloxane copolymers of the general formula ##STR15## where R is an independently selected C.sub.1-13 organic radical; preferably methyl, phenyl or a mixture thereof; R.sup.1 is an arylene radical, preferably phenylene; R.sup.2 is hydrogen or a substituted or unsubstituted alkyl radical, a is an integer from 1 to 12, x, y and z are positive integers, n is an integer which preferably has an average value greater than about 5, and m is a positive integer; which exhibit upon curing, toughness, transparency, self-adhesion, solvent resistance and dirt repellency.